4,894 research outputs found
Temporal Variability of the X-ray Emission of the Crab Nebula Torus
We have analyzed five ROSAT HRI images of the Crab Nebula spanning the years
1991 to 1997 and have found significant changes in the emission structure of
the X-ray torus surrounding the pulsar. Certain regions increase in brightness
by about 20% over the six years, while others show decreases in surface
brightness. The origin of these changes is unclear, but a possible explanation
is that the bulk velocity of the synchrotron radiating electrons has decreased
on the order of 20% as well.Comment: 15 pages plus 6 figures, figure 1 and figure 6 are in color, to
appear in The Astrophysical Journal, Jan 1, 1999, Vol. 510, #
Wind Climatology at 87 km above the Rocky Mountains at Bear Lake Observatory--Fabry-Perot Observations of OH
This paper presents the neutral -wind climatology at approximately 87-km 53 altitude from Utah State University\u27s Bear Lake Observatory (BLO). a mid-latitude site 54 situated in the middle of the Rocky Mountains. The winds were determined using a very 55 sensitive Fabry-Perot interferometer (FPI) observing the OH Me inel (6-2) PI (3) line al 56 843 nm. The climatology. determined from monthly averages of the nightly evolution of 57 the geographic meridional and zonal wind components over forty· five months, has three 58 distinct seasonal patterns: winter (November- February), summer (May-Jul y), and late 59 Slimmer (August and September). The background zonal wind is eastward the whole year 60 except March and April. The background meridional wind is northward in winter and 61 southward during the rest of the year. In late summer. the winds exhibit a very strong 62 semidiurnal tidal variation almost every night. In summer, they exhibit a similar tidal 63 variation on enough nights that a semi diurnal pattern appears in the climatology. In 64 winter. the nighHo·night variability is so great that little structure is evident in the 65 climatology . These winds are compared to those from other techniques or sites: ~l 66 observations from UARS. FPI observations from Michigan, and MF radar observations. 67 While generally agreeing in relative amplitudes and i.n phase. differences do exist. 68 especially the weak semidiurnal tide at BLO in winter and a greatly reduced {tide at spring 69 equinox compared to late summer. It is likely that these differences arise from the 2 70 topographical generation of gravity waves by winds flowing over the Rocky Mountains. 71 The tidal variations are also compared to results from the global-scale wave model 72 (GSWM): our semidiurnal amplitudes arc considerably bigger except in winter, and our 73 phases vary from showing very good agreement in July, fair agreement in April and 74 January, and disagreement in October. These large differences may be evidence that 11011 - 75 linear effects are more important than realized. The behavior of the background winds is 76 consistent with different populations of gravity waves reaching 87 km in summer and 77 winter. The behavior of the semidiurnal tidal variation is consistent\u27 with a strong 78 interaction between the tidal and gravity·wave wind fields, and is consistent with the 79 different summer and Winter gravity wave population s, and with a fall· spring asymmetry 80 characterized by much weaker gravity wave sources in late summer than near spring 81 equinox
Spectra and Light Curves of GRB Afterglows
We performed accurate numerical calculations of angle-, time-, and
frequency-dependent radiative transfer for the relativistic motion of matter in
gamma-ray burst (GRB) models. Our technique for solving the transfer equation,
which is based on the method of characteristics, can be applied to the motion
of matter with a Lorentz factor up to 1000. The effect of synchrotron
self-absorption is taken into account. We computed the spectra and light curves
from electrons with a power-law energy distribution in an expanding
relativistic shock and compare them with available analytic estimates. The
behavior of the optical afterglows from GRB 990510 and GRB 000301c is discussed
qualitatively.Comment: 8 pages, 7 figure
Oxidative phosphorylation and the realkalinization of intracellular pH during recovery from anoxia in Artemia franciscana embryos
The contribution of mitochondrial oxidative phosphorylation to the realkalinization of intracellular pH (pHi) and resynthesis of purine nucleotides during recovery from anoxia was investigated in embryos of Artemia franciscana by assessing the sensitivity of mitochondrial respiration to pH, calculating proton consumption by oxidative phosphorylation, and measuring changes in pHi using 31P nuclear magnetic resonance. Following short-term anoxia, pHi increased from 6.7 to 7.7 during 20 min of aerobic recovery and was temporally correlated with a large increase in ATP. State 3 respiration rates of isolated mitochondria were not substantially compromised at the acidic pH corresponding to the pHi during anoxia (pH 6.3-6.8) compared to values obtained at pH 7.7. Both state 3 respiration rates and respiratory control ratios exhibited broad, substrate-specific pH optima, whereas state 4 respiration rates increased gradually with increasing pH. P:O flux ratios were near their mechanistic limits and did not vary appreciably with pH below 7.5. Estimates of intracellular buffering capacity indicate that between 18 and 37 mmol H+ (1 cytosol)-1 must be consumed to elevate pHi from 6.7 to 7.7. Phosphorylation of mono- and diphosphate purine-nucleotides during the first 20 min of recovery may account for the consumption of up to 4.79 mmol H+ (1 cytosol)-1. An additional 4.77 to 8.18 mmol H+ (1 cytosol)-1 may be consumed through the oxidation of mono- or dicarboxylic acids, respectively, in the Krebs cycle. Taken together, these data are consistent with a role for oxidative phosphorylation in the realkalinization of pHi and resynthesis of purine nucleotides in A. franciscana embryos during recovery from anoxia. © 1995
How are people with mild cognitive impairment or subjective memory complaints managed in primary care? A systematic review
BACKGROUND: Primary care is typically the first point of contact in the health care system for people raising concerns about their memory. However, there is still a lack of high-quality evidence and understanding about how primary care professionals (PCPs) currently manage people at higher risk of developing dementia. OBJECTIVES: To systematically review management strategies provided by PCPs to reduce cognitive decline in people with mild cognitive impairment and subjective memory complaints. METHOD: A systematic search for studies was conducted in December 2019 across five databases (EMBASE, Medline, PsycInfo, CINAHL and Web of Science). Methodological quality of included studies was independently assessed by two authors using the Mixed Methods Appraisal Tool. RESULTS: An initial 11 719 were found, 7250 were screened and 9 studies were included in the review. Most studies were self-reported behaviour surveys. For non-pharmacological strategies, the most frequent advice PCPs provided was to increase physical activity, cognitive stimulation, diet and social stimulation. For pharmacological strategies, PCPs would most frequently not prescribe any treatment. If PCPs did prescribe, the most frequent prescriptions targeted vascular risk factors to reduce the risk of further cognitive decline. CONCLUSION: PCPs reported that they are much more likely to provide non-pharmacological strategies than pharmacological strategies in line with guidelines on preventing the onset of dementia. However, the quality of evidence within the included studies is low and relies on subjective self-reported behaviours. Observational research is needed to provide an accurate reflection of how people with memory problems are managed in primary care
Super-Eddington Atmospheres that Don't Blow Away
We show that magnetized, radiation dominated atmospheres can support steady
state patterns of density inhomogeneity that enable them to radiate at far
above the Eddington limit, without suffering mass loss. The inhomogeneities
consist of periodic shock fronts bounding narrow, high-density regions,
interspersed with much broader regions of low density. The radiation flux
avoids the regions of high density, which are therefore weighed down by
gravity, while gas in the low-density regions is slammed upward into the shock
fronts by radiation force. As the wave pattern moves through the atmosphere,
each parcel of matter alternately experiences upward and downward forces, which
balance on average. Magnetic tension shares the competing forces between
regions of different densities, preventing the atmosphere from blowing apart.
We calculate the density structure and phase speed of the wave pattern, and
relate these to the wavelength, the density contrast, and the factor by which
the net radiation flux exceeds the Eddington limit. In principle, this factor
can be as large as the ratio of magnetic pressure to mean gas pressure, or the
ratio of radiation pressure to gas pressure, whichever is smaller. Although the
magnetic pressure must be large compared to the mean gas pressure in order to
support a large density contrast, it need not be large compared to the
radiation pressure. These highly inhomogeneous flows could represent the
nonlinear development of the "photon bubble" instability discovered by Gammie.
We briefly discuss the applicability of these solutions to astrophysical
systems.Comment: 11 pages, 1 figure, accepted for publication in The Astrophysical
Journa
Evidence for Ordered Magnetic Fields in the Quasar Environment
At a distance of 20 pc from the purported supermassive black hole powering
quasars, temperatures and densities are inferred from optical observations to
be ~10**4 K and ~10**4 cm**-3. Here we present Very Long Baseline
Interferometry radio observations revealing organized magnetic fields on the
parsec scale in the hot plasma surrounding the quasar OQ172 (1442+101). These
magnetic fields rotate the plane of polarization of the radio emission coming
from the core and inner jet of the quasar. The derived rotation measure (RM) is
40,000 rad m**-2 in the rest frame of the quasar. Only 10 mas (a projected
distance of 68 pc) from the nucleus the jet absolute values of RM fall to less
than 100 rad m**-2.Comment: in press at ApJ Letters, 12 page LaTeX document includes 4 postscript
figure
Towards Resolving the Crab Problem: A Linear Accelerator?
Using the exact solution of the axisymmetric pulsar magnetosphere derived in
a previous publication and the conservation laws of the associated MHD flow, we
show that the Lorentz factor of the outflowing plasma increases linearly with
distance from the light cylinder. Therefore, the ratio of the Poynting to
particle energy flux, generically referred to as , decreases inversely
proportional to distance, from a large value (typically \gsim 10^4)near the
light cylinder to \s \simeq 1 at a transistion distance .
Beyond this distance the inertial effects of the outflowing plasma become
important and the magnetic field geometry must deviate from the almost
monopolar form it attains between and . We anticipate
that this is achieved by collimation of the poloidal field lines toward the
rotation axis, ensuring that the magnetic field pressure in the equatorial
region will fall-off faster than ( being the cylindrical radius).
This leads both to a value \s=\ss \ll 1 at the nebular reverse shock at
distance () and to a component of the flow
perpendicular to the equatorial component, as required by observation. The
presence of the strong shock at allows for the efficient conversion
of kinetic energy into radiation. We speculate that the Crab pulsar is unique
in requiring \ss \simeq 3 \times 10^{-3} because of its small translational
velocity, which allowed for the shock distance to grow to values .Comment: 16 pages AASTEX, accepted for publication in the Astrophysical
Journa
The Environmental Dependence of the Infrared Luminosity and Stellar Mass Functions
We investigate the dependence of the galaxy infrared luminosity function (LF)
and the associated stellar mass function (SMF) on environment and spectral type
using photometry from the Two Micron All Sky Survey and redshifts from the Las
Campanas Redshift Survey for galaxies brighter than M_J<-19+5 log h. In the
field environment, galaxies with emission lines have LFs with much steeper
faint end slopes (alpha_J=-1.39) than galaxies without emission lines
(alpha_J=-0.59). In the cluster environment, however, even the non-emission
line galaxies have a steep faint-end LF (alpha_J=-1.22). There is also a
significant (95%) difference between the overall cluster and field LFs, . All of these variations are more
pronounced in the SMFs, which we compute by relating the strength of the 4000 A
break in the optical spectra to a mass-to-light ratio.Comment: 14 pages, 13 figures, emulateapj style ApJ, post-referee. Very minor
changes, mostly typographical in natur
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